1. Field of the Invention
The present invention relates to a control system for a beam deflector and, in particular, to a control system for a Fourier optical processor that positions a deflected scanning beam of light onto a predetermined fixed reference axis.
2. Description of the Prior Art
The application of Fourier optical techniques to signal and image processing is well known. A two dimensional Fourier transform of a scene or image can be obtained with a simple lens. Since every point in the image is acted upon at the same instant in time, this transformation is said to be a parallel, single step operation. This instantaneous image transformation makes the concept of processing data or images in this manner very attractive. Image processing functions can be accomplished using spatial filters in combination with lenses.
A conventional prior art Fourier optical processor generally indicated at 400 in FIG. 1, includes an input optical path 500, a Fourier optical filter 600 and a Fourier optical correlator 700. In such a system the light beam does not move, the input is transformed in a single step, and the processed output is applied to a detector. Small features in the input scene are detectable based upon how much energy is diffracted into the spatial frequency bands where the features are separable.
One or more spatial filters, such as that indicated at 602, and one or more correlation filters, such that indicated at 702, may have either a simple or complex transfer function when used in a Fourier optical processor 400. Spatial filters having only amplitude components in their transfer function are known as "simple" spatial filters. Spatial filters having both amplitude and phase components in their transfer function are referred to as "complex" spatial filters.
The simple spatial filter attenuates certain spatial frequencies and thus enhances certain features of the input image. Complex spatial filters, which can be made using holographic techniques, allow other mathematical operations to be performed in a similar parallel manner. Complex spatial filters of almost any desired transfer function can be realized in practice. This gives optical spatial filters the same broad capabilities as their electrical filter counterparts.
A number of practical difficulties arise which cause problems when attempting to solve real problems with Fourier techniques. Fourier systems are usually difficult to adjust for scale, rotation and other alignment problems. As with any Fourier technique, the energy in the features that are of interest may be quite small and thus cause difficulties in detection and/or classification. There are, however, a number of successful applications of Fourier optical techniques. For example Synthetic Aperture Radar signal processing and similar sonar applications are described in Jensen, H. J., Graham, L. C., Porcello, L. J., Leith, E. N. "Side-looking Airborne Radar", Scientific American, volume 237, pages 84-95, Oct. 1977 and an optical pattern recognition system for the identification of diatoms is described in Almeida, S. P., and Indebetouw, G., "Pattern Recognition via Complex Spatial Filtering", in Stark, H., ed., Applications of Optical Fourier Transforms, Academic Press, 1982, pages 73-81.
Various arrangements have been used to scan a light beam. For example, acousto-optical devices and galvanometer driven, rotationally oscillating mirror light beam scanners, are known. To achieve high precision scanning, servo-controlled galvanometer scanners have been used. U.S. Pat. No. 3,321,766 (Everest) discloses a closed loop galvanometer servo system used in an oscillographic recording apparatus to move a mirror to reflect a beam of light from a fixed source along a path in accordance with a varying applied signal. A portion of the light reflected from the mirror is directed onto an electro-optical potentiometer to produce a beam position signal which is representative of the position of the reflected light beam. The galvanometer is driven in response to the difference between the applied signal and the position signal from the electro-optical potentiometer.
In other scanning applications either separate light beams or coaxial light beams of different wavelengths have been used to produce the reflected beam position signal. Such techniques have been used to position a galvanometer driven mirror for track selection in an optical disk system. See, for example, U.S. Pat. Nos. 4,466,088 and 4,556,964 (both to Trethewey).
The number of problems solveable using Fourier optical techniques could be greatly increased if Fourier optics could be combined with a scanning light beam. Optical processing using spatial filters requires that the pattern of light impinging upon the spatial filter remain stationary so that the beam remain aligned with the optical components, such as spatial filters, correlators and detector arrays. But in a scanning beam system the pattern is not stationary.
The traditional approach to combine scanning with Fourier optics for scanning an object plane has been to use a stationary light beam with a mechanical transport to move the object or medium to be scanned. The use of a two axis positioning stage to mechanically move the medium to be scanned, as described by Das, P. and Ayub, F. M. M., "Fourier Optics and SAW Devices", in Stark, H., ed., Applications of Optical Fourier Transforms, Academic Press, 1982, pages 324-327, has the disadvantages of slow scanning speed, expense and complexity. The use of a rotating optical disk to perform the scanning function as described in Psaltis, D., "Optical disk Based Correlation Architectures", Proceedings of the OSA Topical Meeting on Optical Computing, Feb. 27- Mar. 1, 1989, Optical Society of America, pages 206-209, still requires a moving medium with all of its associated practical difficulties.
When a scanning light beam is used with a stationary object plane the light beam moves in space. To hold the light beam stationary at the detector the scanning motion of the beam must be precisely complemented with a descanning motion.
One prior art example of descanning in the context of optical processing is U.S. Pat. No. 3,879,131 (Cuthbert et al.) which attempts to solve the descanning problem by simultaneously driving two galvanometers, the first to perform the scanning function, and the second to perform the descanning function. The second galvanometer has a motion complementary to the first. A galvanometer driver circuit that seeks to drive both galvanometers in synchronism, but one hundred eighty degrees out of phase. However, nominally identical galvanometers of the same model may differ slightly in their static response, but may differ greatly in their dynamic response to a given input drive signal. To eliminate these differences in response would require either specially selected matched pairs of galvanometers or special compensating circuitry. Moreover, even matched pairs of galvanometers may suffer from different hysteresis and temperature effects which make it very difficult to operate two devices synchronously throughout any reasonable range of operating conditions.
Accordingly, in view of the foregoing it is believed desireable to provide a scanning beam arrangement for use in a Fourier optical processor. It is also believed desireable to provide an arrangement for holding the reflected beam from a rotatable mirror, or the beam deflected from an acousto-optical device, along a stationary axis in space.